[2003]Dissection of the Aorta a New Approach

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VIEWPOINT

Dissection of the aorta: a new approachM Mikich. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Heart2003;89:68

There are two generally proposed causes of dissectionof the aorta: (1) cleavage caused by blood entering thetear; and (2) haemorrhage that dissects the media andtear secondary to the cleavage. Using analysis ofpressures and forces, this article shows that, on someoccasions, these mechanisms alone cannot beresponsible for causing aortic dissection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

This article addresses a possible cause ofdissection of the aorta. Let us start with a

variation of dissection which is not socommonthat is,the dissection that has an entrytear located just beyond the left subclavian artery,but which extends proximally into the ascendingaorta (in other words, the dissection cleavagepropagates from the tear towards the aorticroot).1 2

To simplify matters,let us assume that the bodyis in the supine position. In other positions of thebody the calculation is a bit more complicated butthe result is the same.

ANALYSIS OF PRESSURES AND FORCESForces acting on the flap proximally fromthe tearIt is thought that cleavage from blood entering

through the tear causes the propagation of

dissection towards the aortic root. This is repre-

sented in fig 1. Points denoted with F are in the

false channel and those denoted as T are in the

true channel. If we compare pressure in the true

channel at the tear to pressure at the aortic root it

is obvious that pressure in the true channel

always gradually rises from the tear to the aortic

root (so pressure at T1 is greater than pressure at

the tear, pressure at T2 is greater than pressure at

T1, and pressure at T3 is greater than at T2).

If there is no flow in the false channel the pres-sure at any point within the false channel must bethe same as the pressure at the tear (so pressure

at F0, F1, F2, and F3 is the same as at the tear).

When applying these facts to fig 1 it is obviousthat pressure at point T1 is greater than the pres-

sure at point F1, that pressure at T2 is greater than

pressure at F2, and that pressure at T3 is greaterthan pressure at F3. This means that the pressure

difference pushes the flap towards the adventitia.

Therefore, cleavage by blood from the tear cannotbe the cause of propagation of dissection from the

tear towards the aortic root. This is true no matter

how close the tear is to the aortic root. Even if the

tear is just above the aortic root (as in type I) it isobvious that the cleavage caused by blood from

the tear cannot undermine the aortic valve.

If blood enters from the true channel throughthe tear into the false channel (that is, flows

towards point F3) then pressure at F1 is lower

than pressure at the tear (because liquid can flow

only from higher pressure towards lower pres-

sure), pressure at F2 is lower than at F1, and

pressure at F3 is lower than at F2. Therefore, the

pressure difference that pushes the flap towards

the adventitia is greater than in the case in which

there is no flow in the false channel.

Pressure drop in the aortaTo calculate maximum forces acting on the flap,

the maximum pressure difference between a

point in the false channel and its most adjacentpoint in the true channel (for instance, between

the points F0 and T0) must be determined. The

maximum pressure difference is when there is

maximum pressure drop along the aorta. Since

the maximum pressure at the aortic root is systo-

lic and minimum pressure at the distal end of the

aorta is diastolic, the maximum pressure differ-

ence between the aortic root and distal end

cannot be greater than systolic pressure minus

diastolic pressure. It can also be assumed that

pressure linearly falls from the aortic root to the

distal end.

So if, for example, systolic pressure is

135 mm Hg, diastolic pressure is 90 mm Hg, andthe length of the aorta is 45 cm, then the

maximum pressure drop is (135 90)/45 =1 mm Hg/cm (this pressure drop will be used in

further calculations).

Forces acting on the flap distally from tearIf the distance between the tear and T0 (fig 1) is

1 cm then pressure at T0 is 1 mm Hg lower than

at the tear. When there is no flow in the false

Figure 1 Calculation of forces that act on the flap.Point F0 is just at the distal end of the dissection.

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Correspondence to:Mr Boris Mikich, FranaAlfirevica 29, 10000Zagreb, Croatia;[email protected]. . . . . . . . . . . . . . . . . . . . . . .

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channel, the pressure at F0 (point F0 is just at the distal endof the false channel) is equal to the pressure at the tear, so the

pressure at F0 is 1 mm Hg higher than at T0. Since the

pressure difference at the beginning of the flap (just at the

tear) is zero, the mean pressure difference on the flap distally

from the tear is (0 + 1)/2 = 0.5 mm Hg. If we suppose, for the

purpose of this discussion, that the transverse length of the

flap is 5 cm (longitudinal length is equal to distance between

tear and F0 or T0that is, 1 cm) then the area of the flap dis-

tal to the tear is 5 1 cm2which, when multiplied by the meanpressure difference on the flap, gives a force by which the flap

distal to the tear is pushed towards the true channel.

Converting 0.5 mm Hg to N/m2 gives (1 mmHg = 132.88

N/m2 at 20C): 0.5 132.88 = 66.4 N/m2, and converting cen-

timetres to metres 1 cm = 0.01 m and 5 cm = 0.05 m. Thus,the force is:

66.4 0.01 0.05 = 0.0332 N.

This force is too small to cause any further increase of the

dissection in the distal direction. If blood enters the false

channel then the pressure at F0 is lower than in the case when

there is no flow in the false channel, therefore forces on the

flap are lower too.

Forces from haemorrhageIt is also thought that haemorrhage dissects the media and

that tear is secondary to it. Figure 2 shows a haemorrhage in

the media. Point T is the point in the lumen of the aorta whichis nearest to the haematoma. For the purpose of this

discussion, let us say that the haemorrhage is situated just

beyond the left subclavian artery, about 15 cm from the aortic

root. If we use the same pressure drop as in the previous

example, then pressure at point T is 15 cm 1 mm Hg/cm =15 mm Hg lower than at the aortic root. The maximum possi-

ble pressure in the haematoma is pressure at the aortic root.

This means that pressure in the haematoma cannot be more

than 15 mm Hg higher than at point T and from that the

maximum force produced by the pressure difference between

the haematoma and point T can be calculated.

Let us suppose that the haematoma has an area of 2 2 mm.Converting 15 mm Hg to N/m2 gives: 15 132.88 = 1993 N/m2,

and converting millimetres to metres, 2 mm = 0.002 m. Thus,the maximum force that is produced by the pressure

difference between the haematoma and T is:

1993 0.002 0.002 = 0.00797 N.

That force is too small to make any cleavage in the media orto tear the intima. If there is haemorrhage at the aortic root

then the pressure difference between the haematoma and the

most adjacent point to it in the lumen of the aorta is zero;

therefore it is obvious that there is no force at all and that

blood from that haematoma cannot undermine the aortic

valve.

THE REAL CAUSE OF DISSECTIONFormation of the dissectionIf the aortic smooth muscles contract the adventitia will

follow that contraction. But as seen in fig 3, at the area of the

aorta where the left subclavian, left carotid, and brachio-

cephalic artery originate, downward movement of the aortic

wall is very limited by those arteries. Upward movement of

area A of the aorta is very limited by area D of the aorta. So at

the segment of the aorta which comprises the origin of the left

subclavian artery and area A, the aortic wall is fixed at the

upper and the lower part, so at the upper and lower part of the

segment the adventitia can only partially follow the contrac-

tion of smooth muscles (fig 4). As a consequence high stresses

Figure 2 Calculation of forces that act from the haemorrhage inthe media

Figure 3 Limitation of upwardmovement of area A by area D.

Figure 4 Forces Fs from smooth muscles cause stresses in themedia.

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develop in the media in that segment. If too strong a contrac-

tion of smooth muscles (spasm) occurs, dissection of the

media will take place, especially if there are areas of cystic

degeneration in the media and/or if there is any other

weakness in the media.

Why does dissection take place just beyond the origin of the

left subclavian artery and not at area A? It is because the

smooth muscles of the aorta (and thus also forces from the

smooth muscles) are not arranged around the originof the left

subclavian artery as evenly as they are at area A. This uneven

distribution causes the concentration of forces in the aortic

media at the origin of the left subclavian artery which then

causes dissection.

Eventually, if the forces from the smooth muscles continue

to increase, the intima and part of the media on the smooth

muscle will break and a tear will result.

Propagation of dissectionFigure 5 shows a transverse cross section of dissection through

the flap. It can be seen that blood pressure forces act on all the

inside surfaces of the false and true channels. Assuming, for

simplicitys sake, that pressures both in the true and in the

false channel are the same, the forces acting on the flap from

the false channel and from the true channel are also equal(and so cancel each other) and therefore they are not shown.

The forces shown stretch the aortic wall. While stretching,

the aortic wall stretches the flap in the direction that is

perpendicular to the long axis of the aorta and parallel with

the flap. Because each force must have a counter force of the

same intensity and opposite direction it means that the flap

pulls the aortic wall by force Ff (fig 5, right). That force is

caused by elasticity of the flap and/or the presence of forces

from smooth muscles in the flap. This force Ff by which the

flap pulls the aortic wall tends to damage the media further

and to increase dissection in the transverse direction. The

increasing dissection in the transverse direction is accompa-

nied by movement of the flap towards the centre of the aorta.

This movement pulls the rest of the flap which is located dis-tally and/or proximally from the part of the flap shown in cross

section, and that causes further dissection in the distal and/or

proximal direction.

If blood pressure rises the stretching of the flap increases;

consequently the forces that tend to damage the media and to

increase dissection also rise. Once the flap, which is

perpendicular to the long axis of the aorta, reaches a certain

length (the length depending on pressure in the aorta), the

elastic forces from the flap alone are sufficient to increase dis-

section further.

If a tear does not occur, blood from haemorrhage could

slowly fill the false channel, thus establishing in the false

channel the same pressure as in the aorta, as occurs when

there is a tear. But, because of low inflow of blood from haem-orrhage in this case, dissection can propagate only very slowly.

CONCLUSIONDissection takes place as a consequence of major contraction

of the smooth muscles, especially when there are areas of

cystic degeneration in the media, and haemorrhage takes

place because of dissection of the media. Administration ofblockers and/or calcium antagonists will reduce this strong

contraction of the smooth muscles, thus reducing dissection.3

On the other hand, discontinuation of long term therapy with

blockers may provoke dissection.4

Fibrous tissue at the aortic root also fixes the aortic wall,

which makes this part of the aorta also suitable for initiating

dissection. This fixing of the aortic wall explains why morethan 95% of dissections begin at two places: either just above

the aortic valve or just beyond the origin of the left subclavian

artery.5

In hypertensives there is an increase in smooth muscle

mass and that also produces a stronger force.6 7 That, together

with acquired cystic degeneration in hypertensives, mayexplain why dissection takes place mostly in hypertensives.

Smooth muscles in the media in large, elastic arteries are

generally helically oriented.8 Therefore it is readily possiblethat dissection propagates in a direction perpendicular to the

orientation of these helically oriented smooth muscles

(analogous to normal propagation which is perpendicular tocircumferentially oriented smooth muscles). As a result spiral

dissection develops, known as spiral barber pole dissection.1

REFERENCES1 Schlant RC, Alexander RW.Hursts the heart, 8th edn. New York:

McGraw-Hill, 1994:2170.2 Hurst JW, Alpert JS.Diagnostic atlas of the heart. New York: Raven

Press, 1994:337.3 Hoshino T, Ohmae M, Sakai A. Spontaneous resolution of a dissection

of the descending aorta after medical treatment with a beta blocker anda calcium antagonist.Br Heart J1987;58:824.

4 Eber B, Tscheliessnigg KH, Anelli-Monti M,et al.Aortic dissection due todiscontinuation of beta-blocker therapy.Cardiology1993;83:12831.

5 Hurst JW, Alpert JS.Diagnostic atlas of the heart. New York: RavenPress, 1994:338.

6 Owens GK, Schwartz SM. Alterations in vascular smooth muscle mass inthe spontanteneously hypertensive rat. Role in cellular hypertrophy,hyperploidy and hyperplasia.Circ Res1982;51:2809.

7 Owens GK, Schwartz SM. Vascular smooth muscle cell hypertrophy andhyperploidy in the Goldblatt hypertensive rat. Circ Res1983;53:491501.

8 Schlant RC,Alexander RW. Hursts the heart, 8th ed. New York:McGraw-Hill, 1994:31.

Figure 5 Transverse (perpendicular to the long axis of the aorta)cross section of a dissection. The left and right cross sections areidentical, but in the right cross section the flap is not present so thatforces Ff can be shown. Forces from blood pressure acting on theflap from the false channel and the true channel are equal andcancel each other, and therefore they are not shown. Fp, forces fromblood pressure in the aorta; Ff, forces with which the flap pulls theaortic wall.

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[2003]Dissection of the Aorta a New Approach